1. Field of the Invention
The present invention relates to a porous Ti-Al alloy which has a large specific surface area and which is suitable for capacitor anode bodies, catalyzers, filters, etc., and also to a method for producing the same.
2. Description of the Prior Art
The use of porous sintered bodies of metal for capacitor anode bodies, catalyzers, fuel filters for aircrafts, etc., is known. In such applications it is necessary for the sintered body to have large specific surface area and porosity and a certain amount of mechanical strength. When used as a capacitor anode body, in addition to the above requirements, it is further necessary for the sintered body to have a valving effect. Aluminum, tantalum, niobium, titanium, etc. are known as metals having such a valving effect.
These valving metals, as for example aluminum, have the disadvantage that when a capacitor anode body is produced by press-shaping the aluminum powder and sintering the shaped body, increasing in the pressing force or the sintering temperature for the purpose of giving the sintered body sufficient mechanical strength to withstand handling during production and utilization, reduces the specific surface area and lowers the capacitance of the resulting capacitor. Such a disadvantage is also true for tantalum, niobium, titanium, etc. A similar problem exists when making a filter from the porous sintered body. For instance, when manufacturing a filter for adsorbing oxygen gas, hydrogen gas, etc., active metals such as aluminum, titanium, etc. are used as metals for effecting the adsorptive reaction, but similarly to the case of the above-described capacitor anode body it is difficult to produce a sintered body consisting of aluminum only or titanium only and yet having a sufficient mechanical strength and a large specific surface area.
German Patent Application No. P 26 36 279.8 published before examination discloses a method for producing a porous anode body for use in a capacitor by mixing and press-shaping tantalum powder and aluminum powder and then heat-treating the press-shaped body of the mixture above the softening temperature of aluminum. However, its description is very brief, and no description is made at all with respect to detailed manufacturing conditions such as, for example, temperature range and period of the heat treatment as well as the characteristics of the produced porous Ta-Al anode body. Moreover, use of tantalum should be avoided, because it is expensive.
On the other hand, Japanese Patent Application No. 51-10165 published before examination discloses a method for producing aluminum-titanium alloy powder. According to its disclosure, mixed powder of aluminum and titanium is press-shaped, and then sintered to form an alloy by heating at a temperature where the press-shaped body partly becomes liquid phase, that is, at a temperature (for instance, 700.degree..about.1000.degree. C.) higher than the melting point of aluminum. Once the alloy is formed and thereafter the sintered body is crushed to produce Al-Ti alloy powder. However, the described technique is not specifically intended to produce a porous body, but is merely a method for producing Al-Ti alloy powder.
In general, a sintering process involving a liquid phase tends to result in densification of the porous body due to the surface tension of the liquid and the resultant porous structure is such that the pores tend to be isolated in the body, that is, the sintered body has a low specific surface area, although such a porous body may be easily crushed into a form of powder. Therefore, a Ti-Al sintered body produced according to the teaching of this prior art reference would have a high density and a low specific surface area, so that it has the disadvantage that when it is used as a material for capacitor anode bodies or the like, the capacitance of the produced capacitor is too small.
U.S. Pat. No. 3,599,053 discloses a method for making a porous sintered body of Ti-Al alloy and its application to capacitor. However, a porous sintered body according to this prior art is produced through a lengthy process of forming Ti-Al alloy by a vacuum melting or arc melting of a mixture of Ti powder and Al powder, crushing the Ti-Al alloy into fine particles, press-shaping the Ti-Al alloy powder, and sintering the shaped alloy powder at a temperature of 800.degree. to 1300.degree. C. in a nonoxidizing atmosphere. This process has the following disadvantages: First, the process of making the porous body is very complex because of inclusion of crushing and two heat treatments. Second, when Ti-Al alloys (TiAl.sub.3) are crushed into fine particles, Ti is partially exposed at the surface of the crushed TiAl.sub.3 particles. With such Ti-exposed surfaces, the sintered body, when used in a capacitor, shows an undesirably large leakage current. In order not to expose Ti at the surface of the sintered body, a lengthy heat treatment is necessary to fully cover the exposed Ti surface with Al of other TiAl.sub.3 particles. Third, porous sintered bodies having large specific surface area and small density can not be produced, because the heat treatment is performed at a temperature higher than the melting point of Al.